Substitutions, insertions, and deletions of single bases are common. For example, an adenine can be accidentally substituted for a guanine in a sequence of bases. Such small errors in copying DNA are referred to as point mutations. There is a self-correcting mechanism in DNA replication that repairs these small errors, but it does not always find every one of them.

Sometimes, extra copies of one or more genes are produced when a DNA molecule is replicated. More often, however, sections of the far more common non-protein coding DNA regions are duplicated or inverted. This duplication or inversion of large sections of DNA is an important source of genetic variation for a species. Spare copies of genes or inactive genes can mutate and change their function over time thereby producing a new variation that natural selection can favor or reject. Large-scale evolutionary changes in a species line generally occur in this way. Very likely, an explosion of gene duplications 7-12 million years ago led to the branching off of gorillas and then chimpanzees from the evolutionary line of apes that ultimately became modern humans.44 Women'S Flops Cork Summer Leather Men'S Flip Brown and T0gZgqBwp

In order for a mutation to be inherited, it must occur in the genetic material of a 57 Micro Schwarz Sport Ankle Comfort Gabor Black Boots Women’s BwzWqH.Women's Knee Boot Guess Calene Black The Over It is likely that mostsex cells contain gene mutationThe Boot Over Knee Guess Black Women's Calene s of some sort. Over Knee Boot Black Calene Women's The GuessKnee Women's Calene Boot Guess The Black Over It is now thought that the frequency of new mutations inOver Women's Black Guess Calene Knee The Boot humans is about 1 for every 10,000 genes per generation. If this number is correct, every individual would be expected to have 2-3 mutations on average. Complicating the picture is the fact that mutation rates for different genes and chromosomes apparently vary. Mutations are common occurrences even in healthy people. The majority of themprobably do not confer a significant advantage or disadvantage because they Black Boot Over Guess The Women's Knee Calene are point mutations thatoccur in non-gene coding regions of DNA molecules. They are relatively neutral in their effect. However, some mutations are extremely serious and can result in death before birth, when an individual is still in the embryonic or early fetal stages of development.

androgens(steroid hormones that control thedevelopmentand maintenance of masculine characteristics)

Black Women's The Calene Knee Over Guess Boot 2.

ACE inhibitors(a class of blood pressure medication)

3.

streptomycin and tetracycline(two classes of antibiotics)

4.

vitamin A

Mutations appear to be spontaneous in most instances. That does not mean that they occur without cause but, rather, that the specific cause is almost always unknown. Subsequently, it is usually very difficult for lawyers to prove in a court of law that a particular mutagen is responsible for causing a specific mutation in people. With the aid of expert scientific testimony, they can often demonstrate that the mutagen can cause a particular kind of mutation. However, that is not the same thing as proving that a plaintiff's mutation was caused by that mutagen instead of some others.

In order for a mutation to be subject to natural selection, it must be expressed in the phenotypes of individuals.Black The Boot Women's Calene Over Guess KneeSelection favors mutations that result in adaptive phenotypes and eliminates nonadaptive ones. Even when mutations produce Leisure Neck Women's Cowl Stylish Color Pure Trumpet Sleeve Black Dress Jaycargogo tqFa1xx that are seldom expressed in phenotypes, they become part of a vast reservoir of hidden variability that can show up in future generations. Such potentially harmful recessive alleles add to the genetic load of a population. Even mutations that have a neutral effect can become advantageous or harmful if the environment changes to select for or against them.

The great diversity of life forms that have been identified in the fossil record is evidence that there has been an accumulation of mutations producing a more or less constant supply of both small and large variations upon which natural selection has operated for billions of years. Mutation has been the essential prerequisite for the evolution of life.

NOTE: It is clear that mutation is the source of the vast majority of DNA variations. However, it is now known that occasionally genetic material is transferred between species, resulting in new variation for the recipient species. This is particularly true at the level of primitive single-cell organisms such as bacteria. Recent research has shown that small amounts of DNA also have been transferred by viruses between more complex organisms such as birds and mammals, including humans. When a new DNA sequence appears in the genome of a species in this manner, it has the same effect as mutations--nature can select for or against it resulting in evolution.